1. Field of the Invention
The present invention relates to a line switching system for switching a communication line smoothly that is transmitting data when plural types of communication lines can be used in a local area network such as a home LAN. The present invention also relates to a server and a terminal of the system.
2. Description of the Prior Art
In recent years, computer networks become commonplace, and especially the Internet through which various world wide network services can be obtained easily has become significantly widespread. As an access line to the Internet, broadband lines have gained the spotlight, which include an asymmetric digital subscriber line (ADSL) that uses a usual telephone line, a cable television (CATV) and a fiber to the home (FTTH) that uses an optical fiber. There are increasing number of people who use the service for high speed connection to the Internet utilizing the broadband line and download contents (streaming information) such as a music or a video via the Internet for listening or viewing.
Moreover, it is also common to build a home network (or a home LAN) that connects plural terminals (e.g. personal computers) at home using Ethernet (a trademark), a wireless LAN, power-line LAN or others, so that plural terminals can connect to the Internet via a router (or a home server). In such a home network, a certain line can be hardly used in a certain place within a house.
For example, when using a portable terminal such as a note type personal computer with moving within a house, the terminal can be connected to the home network via the Ethernet (a trademark) line in a certain place. In another place, however, it can happen that an Ethernet (a trademark) cable cannot be laid so that another line (e.g., a wireless line) should be used. In this case, a technique for switching from the Ethernet (a trademark) line to the wireless line is necessary.
A conventional example of such a line switching technique in a local area network (LAN) is disclosed as a communication terminal in Japanese unexamined patent publication No. 9-305508. In this device, a terminal detects a mating or unmating of a communication cable or a connection state in communication without user's operation of the terminal, so as to switch a network connection interface at the terminal side automatically.
As another conventional example, there is a network connection device that is described in Japanese unexamined patent publication No. 2000-261468. This device switches a method for transmitting video information in accordance with a communication interface of another device that is connected to the device in a video information network using an interface of IEEE1394.
FIG. 13 is block diagram of a conventional line switching system in a usual home network. In this example, two cable communication lines 103 and 104 and one wireless communication line 105 are provided as communication interfaces (communication lines) between a home server 101 and a terminal (e.g., a note type personal computer) 102. For example, the first cable communication line 103 is a communication line utilizing a power-line carrier, the second cable communication line 104 is an Ethernet (a trademark) line using an unshielded twisted pair (UTP) cable, and the wireless communication line 105 is a wireless communication line based on the standard IEEE802.11.
The home server 101 comprises a cable mate detecting portion 101a for detecting a mating or unmating of a communication cable and a switch processing portion 101b for switching lines in accordance with the detection result. The terminal 102 also comprises a cable mate detecting portion 102a and a switch processing portion 102b. 
In the conventional line switching method, when the terminal 102 moves, the terminal 102 determines which line should be used after switching. Therefore, the terminal 102 has to transmit a request to resend the data to the home server 101 using the line after switching. There is a problem that if the terminal 102 was receiving streaming information such as a video, the video is paused when the line is switched.
There is another problem that it is difficult that the home server 101 switches the line in synchronization with the terminal 102 side, so a load of an application will be increased for the processing the request to resend the streaming information, which was received from the terminal 102 when the line is switched.
The streaming information that is transmitted to a terminal via a home server (a router) does not flow in all lines but flows only in the line that the terminal is connected originally for using bands efficiently. Therefore, it is important that if the line connected to the terminal is switched, the home server (may be referred to simply as a server) must respond to the switching and switch the line for sending the streaming information smoothly.
For example, if a communication speed changes (especially to a low speed) due to a line switching in a home network, it is necessary to consider the influence of the speed change so that streaming information can be received correctly. It is also necessary to design so that the streaming information is not dropped out corresponding to the time period necessary for the switching.